LTE-Advanced (LTE-A) system improves spectrum efficiency by utilizing a diverse set of base stations deployed in a heterogeneous network topology. Using a mixture of macro, pico, femto and relay base stations, heterogeneous networks enable flexible and low-cost deployments and provide a uniform broadband user experience. In a heterogeneous network, smarter resource coordination among base stations, better base station selection strategies and more advance techniques for efficient interference management can provide substantial gains in throughput and user experience as compared to a conventional homogeneous network. For example, coordinated multiple points (CoMP), also known as multi-BS/site MIMO, is used to enhance the performance of cell-edge UEs in LTE-Advanced systems.
In 3GPP RAN1#65, the issue of downlink control capacity was first discussed for CoMP scenario 4, where both macrocell base station and remote radio heads (RRH) inside the macrocell coverage share the same physical cell ID. In 3GPP RAN1#66, it was agreed as a working assumption to have a new physical control channel inside the region of legacy physical downlink shared channel (PDSCH). The main benefits to have this new physical control channel are for the better support of HetNet, CoMP, and MU-MIMO. In 3GPP RAN1#68, it was agreed that an enhanced physical downlink control channel (ePDCCH) spans both first and second slots in the region of legacy PDSCH.
To exploit both diversity and beamforming/scheduling gain in ePDCCH, both distributed and localized transmission schemes are supported. However, supporting both distributed and localized transmission in both common and UE-specific search spaces for each UE may result in excessive control signaling and increased number of blind decoding. Furthermore, with existing search space design for legacy PDCCH, UE may not be able to enjoy the beamforming and/or scheduling gain in a specific frequency sub-band due to the predefined mapping rule from the physical resource to search space. To achieve all these goals, a solution to aggregate the assigned physical radio resources for both distributed and localized transmission schemes of ePDCCH and configure common and UE-specific search space for each UE is sought.